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Human evolution driven by climate change

Mark Maslin co-authors presentation of new evidence

New research, published on 16th October in the journal PLOS ONE, shows that early human evolution was driven by short pulses of rapid environmental change in East Africa.

Professor Mark Maslin (UCL Geography) and Dr Susanne Shultz (University of Manchester, Life Sciences) show for the first time the link between the waxing and waning of huge lakes in the East African Rift valley and the brain expansion and migration of the early human species.

As Mark explains: “It seems modern humans were born from climate change, as they had to deal with rapid switching from famine to feast - and back again. This drove the appearance of new species with bigger brains and also pushed them out of East Africa into Eurasia and South Africa.”

Susanne adds, “We have long recognised that many key events in human evolution, including the appearance of modern humans, occurred in East Africa. Our study highlights how important the Rift Valley climate was in driving the evolution of our species.”

The researchers compiled all the known occurrence of lakes over the last 5 million years, from the North of Ethiopia down to Tanzania. Using statistical modelling, the team compared the lake and climate records with evidence of human evolution, providing the strongest evidence to date for the Pulse Climate Variability hypothesis.

Originally proposed by Maslin and Trauth in 2009, the theory states that at 2.6, 1.8 and 1 million years ago there were short periods of around 200,000 years when East Africa became very sensitive to changes in the Earth's orbit, resulting in rapid cycling between very dry and then wet periods of about 20,000 years.

As Mark explains: “Due to these changes in orbit, the climate of East Africa seems to go through extreme oscillations from having huge deep freshwater lakes surrounded by rich lush vegetation to extremely arid conditions - like today - with sand dunes in the floor of the Rift Valley. These changes resulted in the evolution of a new species with bigger brains, and also forced early humans to disperse out of East Africa.”

According to Susanne: “We found that around 1.9 million years ago a number of new species appeared, which we believe is directly related to new ecological conditions in the East African Rift Valley, in particular the appearance of deep-freshwater lakes. Among these species was early Homo erectus with a brain 80% bigger than their predecessors..”

After this massive jump, smaller steps in brain expansion in Africa were driven by regional aridity, demonstrating the complexity of the evolution of modern humans.

The major migrations of hominins out of the Rift Valley appear to have happened during very wet times. When the basins were filled with water there would have been limited space, but the expanding populations would have been able to follow the Nile tributaries northwards.